1. Field of the Invention
This invention generally relates to a beam-combining laser beam source device wherein laser beams produced by a plurality of laser beam sources, such as semiconductor lasers which have a low power output, are combined so as to multiply the power of the individual laser beam sources. This invention particularly relates to a beam-combining laser beam source device wherein a high accuracy of laser beam combination is kept consistent.
2. Description of the Prior Art
Light beam scanning apparatuses wherein a laser beam is deflected by a light deflector in order to scan a surface with the laser beam have heretofore been widely used in, for example, scanning recording apparatuses and scanning read-out apparatuses. In such light beam scanning apparatuses, it is desired that a plurality of laser beams be combined so as to obtain laser beams having a high intensity and that those laser beams having a high intensity be used as scanning light in order to, for example, increase the speed at which the information recorded on a surface to be scanned is recorded or read out. Combination of the laser beams is required particularly when semiconductor lasers are used as the laser beam sources. Specifically, a semiconductor laser has various advantages over a gas laser or the like in that the semiconductor laser is small in size, cheap and consumes little power, and that the laser beam can be modulated directly when the drive current applied to the semiconductor laser is changed.
However, the output power of the semiconductor laser is low (20 mW to 30 mW) when the semiconductor laser is operated in order to continuously radiate the laser beam. Therefore, the semiconductor laser is not suitable for use in a light beam scanning apparatus wherein scanning light having a high energy is necessary, for example: a scanning recording apparatus wherein information is recorded on a recording material which has a low sensitivity, such as a draw material (a metal film, an amorphous film, or the like).
On the other hand, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored during exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor. In U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395, use of a stimulable phosphor in a radiation image recording and reproducing system was proposed. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object such as the human body in order to store a radiation image of the object thereon, and is then scanned with a stimulating ray such as a laser beam which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted into an electric image signal, and the image signal is used to reproduce the radiation image of the object as a visible image on a recording material such as photographic film, a display device such as a cathode ray tube (CRT), or the like.
In the aforesaid radiation image recording and reproducing system, it is desired to use a light beam scanning apparatus, wherein a semiconductor laser is used, in order to scan the stimulable phosphor sheet on which a radiation image has been stored, thereby to read out the radiation image. However, in order to quickly read out the radiation image from the stimulable phosphor sheet, it is necessary to irradiate stimulating rays which have a sufficiently high energy onto the stimulable phosphor sheet. Accordingly, it is not always possible to use a light beam scanning apparatus, wherein a semiconductor laser is used, in order to read out an image in the radiation image recording and reproducing system.
In order to obtain a scanning laser beam having a sufficiently high energy from a semiconductor laser, or the similar lasers, having a low power outputs, a plurality of laser beam sources may be used, and laser beams radiated from the laser beam sources may be combined so as to multiply the power of the individual laser beam sources.
In general, in order to combine the laser beams produced by a plurality of laser beam sources, the laser beams produced by the laser beam sources are collimated respectively by collimator lenses, guided along optical paths parallel and close to one another, and converged to a common spot by a converging lens. For example, in Japanese Patent Application No. 63(1988)-35836 a beam-combining laser beam source unit is provided with a plurality of laser beam sources and which efficiently combines laser beams. The proposed laser beam source unit comprises a plurality of laser beam sources, collimator optical systems positioned in optical paths of laser beams, which are radiated from the laser beam sources, in order to collimate the laser beams, and optical path adjusting elements respectively positioned in the optical paths of the laser beams in order to radiate the laser beams along optical paths parallel and close to one another. The laser beam sources, the collimator optical systems, and the optical path adjusting elements are supported on a single support means. With the proposed laser beam source unit, when the laser beams radiated from the laser beam source unit are caused to impinge upon a converging lens, the laser beams can be converged to a single spot at a desired position.
In order to accurately combine the laser beams in the laser beam source unit described above, it is necessary to accurately adjust the positions of the optical elements of the laser beam source unit so that the laser beams radiated from the laser beam source unit are collimated and follow predetermined optical paths which are parallel to one another. For this purpose, it is necessary to adjust the temperature of the whole laser beam source unit so that it is uniform. Specifically, if the temperature of the laser beam source unit were not uniform, the holding means would be deformed, and therefore the accuracy of the positions of the collimator optical systems and the optical path adjusting elements would become deteriorated. As a result, the condition in which the laser beams are radiated from the laser beam source unit varies, and the laser beams cannot be combined accurately at a predetermined position. For example, in cases where the laser beam source unit is used as a means to generate scanning light which is used to read out or reproduce a radiation image in the radiation image recording and reproducing system described above, scanning light comprising the laser beams combined in a desirable manner could not be obtained if a temperature difference on the order of approximately 1.degree. C. arose with the laser beam source unit.
In order to make the temperature of the laser beam source unit uniform, providing the laser beam source unit with heaters and a temperature sensor, and to control the operations of the heaters based on the detection of the temperature sensor so that the temperature of the laser beam source unit is kept at a constant value (for example, 48.degree. C.) are considered.
However, in general, the temperature of ambient air around the laser beam source unit which is being operated is lower than the temperature of the laser beam source unit. Therefore, in cases where the laser beam source unit is located so that it is in contact with ambient air, even though the temperature of the laser beam source unit is controlled in the manner described above, the laser beam source unit is cooled by the ambient air, so that the temperature of the laser beam source unit in positions close to the heaters and positions remote therefrom.